1. Field of the Invention
The present invention relates to a Liquid Crystal Display (LCD) backlight apparatus, and more particularly, to an LCD backlight apparatus, which has a scattering pattern formed in the entire bottom surface of a light guide plate and a monochromatic light source for emitting light in a specific beam angle placed at a side of the light guide plate in order to reduce the Bezel width without increasing the thickness of an LCD, and an LCD having said LCD backlight.
2. Description of the Related Art
An LCD includes an LCD panel for verifying light transmittance in response to input electric signals to display various images, a circuit board for applying the electric signals to the LCD panel, a backlight apparatus for illuminating the LCD panel and a housing for enclosing these components.
The LCD panel generally has a pair of opposed substrates, crystal liquid interposed between the both substrates and pixel and common electrodes formed in opposed faces of the substrates, in which voltage is variably applied to the pixel and common electrodes to artificially re-arrange the orientation of liquid crystal molecules between the substrates and thus vary the transmittance of light thereby displaying various images.
An LCD backlight apparatus illuminates an LCD panel of an LCD via direct illumination or side-emitting illumination. In the direct illumination, light is projected onto the LCD panel directly from an underlying light source. In the side-emitting illumination, light from a light source is emitted in lateral directions and then re-directed upward via a reflective plate or a scattering pattern to illuminate the LCD panel.
An LCD backlight apparatus adopting the side-emitting illumination uses at least one set of RGB light sources to form white light. Generally, one RGB light source set includes four Light Emitting Diodes (LEDs) which are arranged in the order of B, G, R and G. In this case, it is required to precisely control light beams emitted from the respective LEDs to mix together before reaching the scattering pattern.
More detailed description will be provided with reference to FIGS. 1 and 2, in which FIG. 1 is a plan view illustrating the operation of a conventional LCD backlight apparatus, and FIG. 2 is a vertical cross-sectional view illustrating the operation of the conventional LCD backlight apparatus.
As shown in FIGS. 1 and 2, a conventional side-emitting backlight apparatus 10 is placed under an LCD panel (not shown) The backlight apparatus 10 includes a light guide plate 12 having a scattering pattern 14 in the underside and a plurality of LEDs 18 placed in support plates 16 at both sides of the light guide plate 12, in which the LEDs 18 are spaced from adjacent ones to a predetermined pitch P. An LED set includes B, G, R and G LEDs 18 (or RGB LEDs 18) to form white light, and functions as a light source of the LCD.
Each LED 18 emits light within an angle θ when seen in the plan view and an angle α when seen in the vertical cross-sectional view, which are referred to as “beam angles.” The LED 18 has upward and downward beam angles typically in the range of ±15°.
Referring to FIG. 1, B, G, R and G light beams L are mixed together to form white light at a point P1 distanced from the light source, in which the distance from the light source, that is, the LEDs 18 to the point P1 will be referred to as a “reference length l.” The reference length l is determined by the planar beam angle θ of the LEDs 18 and the interval of the LEDs 18, and expressed in Equation 1 below:l=k×P/tan(θ/2)  Equation 1,
wherein k is coefficient of correlation.
In this case, it is required that the B, G, R and G light beams not collide against the scattering pattern 14 before propagating the reference length l. If the beams are reflected from the scattering pattern 20 before propagating the reference length l, partial RGB beams are mixed insufficient to form white light and therefore cause defective illumination such as a rainbow or hot spots when projected onto the LCD panel 20.
Therefore, the light guide plate 12 has patternless first areas 12A, which are extended inward to a predetermined patternless width l1 from both edges of the light guide plate 12, and a patterned second area 12B formed between the first areas 12A. The first areas 12A each have a smooth reflecting surface without any scattering pattern, but the second area 12B is provided with the scattering pattern 14 to reflect mixed white light toward the LCD panel 20. In this case, the patternless width l1 is determined by the reference length l of the LED 18 and the height h of the LED 18 from the bottom surface of the light guide plate 12 as expressed in Equation 2 below:l1=√{square root over (l2−h2)}  Equation 2.
That is, according to the arrangement of the scattering pattern 14 of the light guide plate 12 and the LEDs 18, the beeline of the LEDs 18 each to the scattering pattern 14 becomes the reference length l so that the light beams can reach the scattering pattern 14 only after being mixed into white light.
Describing the propagation and reflection of light by the above arrangement in more detail, first RGB beams L1 emitted at an upper beam angle α collides against the upper surface of the light guide plate 12 and is directed toward the bottom of the light guide plate 12 via internal reflection. The RGB beams L1 are mixed together to form white light while propagating to the bottom surface, and then projected toward the LCD panel 20 by the scattering pattern 14 in the bottom surface of the light guide plate 12.
Second RGB beams L2 emitted within the upper beam angle α are re-directed toward the LCD panel 20 via a process similar to that of the RGB beams L1.
Third RGB beams L3 emitted in an upper beam angle smaller than that of the second RGB beams L2 reach the first area 12A opposite to the light source thereof, and reflect plural times in the second area 12A before being projected by the scattering pattern 14 toward the LCD panel 20.
Fourth RGB beams L4 emitted within a downward beam angle a are mixed together forming white light when have directly reached the scattering pattern 14, and then projected by the scattering pattern 14 toward the LCD panel 20.
In the meantime, fifth RGB beams L5 emitted at a downward beam angle α are reflected sequentially by the bottom and upper surfaces of the light guide plate 12 before being projected by the scattering pattern 14 toward the LCD panel 20.
Accordingly, the conventional backlight apparatus 10 is required to ensure a “Bezel width” corresponding to the patternless width l1 in addition to the surface size of the LCD panel 20. Undesirably, the Bezel width increases the surface size of the LCD when LCD panel size or LCD screen size is fixed. The patternless width l1 is at least the Bezel width, and the patternless width and the Bezel width will be used equivalently in the specification unless specifically mentioned otherwise.
As an approach to reduce the Bezel width, the LEDs 18 as the RGB light source may be placed closer to reduce the pitch P. However, this approach disadvantageously raises the price of the backlight apparatus and an LCD as a final product.
An LCD backlight apparatus as shown in FIG. 3 has been proposed to overcome the foregoing disadvantages of the prior art.
Referring to a vertical cross-sectional view in FIG. 3, a conventional LCD backlight apparatus 30 includes an RGB light source 32 comprised of a plurality of LEDs, a first reflector 34 for laterally redirecting light beams L emitted from the RGB light source, a light guide 36 connected at one end to the first reflector 34 to guide the redirected light beams L to the other end thereof, a second reflector 38 attached by an input side to the other end of the light guide 36 to direct the light beams L in an opposite direction and a light guide plate 40 connected to an output side of the second reflector 38. The light guide plate 40 has a scattering pattern 42 in the bottom and a smooth transparent surface in the top. Since the top surface of the light guide plate 40 internally reflects light within the range of a predetermined angle, the light beams L introduced into the light guide plate 40 are reflected from the scattering pattern 42 directly or after being internally reflected by the upper surface of the light guide plate 40 so as to be projected toward an LCD panel (not shown) above the light guide plate 40.
This eliminates the necessity of ensuring the Bezel width to the extent of the patternless width l1 as in FIGS. 1 and 2, and thus the surface area of the LCD can be advantageously reduced.
However, the light guide 36 and the light source 32 are provided in the rear surface of the LCD, thereby disadvantageously increasing the thickness of the LCD.
Accordingly, a novel approach capable of decreasing the Bezel width without increasing the thickness of the LCD has been required.